Clamp-on type probes have been developed and heretofor utilized in electrical current measurement apparatus. These devices are designed to permit positioning of the probe onto a current carrying conductor and thereby avoid disturbance of the conductor itself, such as that required by cutting and inserting a shunt or other precise current measuring devices. The clamp-on type probes of the prior art generally comprise two C-shaped sections of laminated sheets of metal that are designed to have the desired magnetic characteristics. Each of the C-shaped sections is supported in a structure which may be fabricated from a non-magnetic material that is configured as a pliers or scissors to permit relative movement of the two core sections for opening and closing of the abutting ends of the two sections. In the current measuring probes of the type with which this invention is concerned, a Hall-effect device is often utilized as the magnetic field sensing element. This Hall-effect device, in accordance with the prior art, is often positioned at an end-face of one of the C-shaped core sections and the necessary electrical conductors for applying the driving current and detecting the resultant Hall-effect generated voltage are extended from the device and along the core through the supporting structure to an appropriate voltage measuring instrument. The voltage measuring instrument is usually provided with an indicating meter which provides a relative indication of the current carried by a conductor that extends through the magnetic core of the probe.
A substantial disadvantage and defect of the prior art magnetic core construction for such clamp-on probes has been the inability of these devices to compensate for or eliminate the hysteresis effect of magnetic materials. Since the object of such measuring devices is to determine or ascertain the current carried by a conductor at a particular instant, it is apparent that the current being measured will probably change over a period of time. The relationship of this current change to the measured indication is affected by the hysteresis characteristic of the magnetic core material and reduces the accuracy of the indication of the current that is carried by a conductor. As a consequence of the hysteresis effect, there will be a difference in the current indicated by the instrument for a same specific current carried by the conductor depending on whether the current has increased from a previously measured point or has decreased from a previously measured point. This difference is a function of the hysteresis characteristics of the particular magnetic core material.
In an attempt to eliminate or at least minimize the hysteresis effect, prior art devices have utilized magnetic materials which are designed to have a more idealized hysteresis characteristic. That is, materials for the core are designed or selected to have the hysteresis envelope reduced to a minimum and thereby attempt to reduce the errors introduced as a consequence of changing magnetic fields such as are produced by an increase or decrease in measured current. This technique of selecting specifically magnetic materials has been moderately effective where the hysteresis characteristics become more idealized but the effect remains and does produce an error in the current measurements. It will also be apparent that selection of specific magnetic materials having the desired magnetic characteristic results in an added expense which substantially increases the cost of the current measuring equipment.
Although selection of magnetic materials having the most advantageous hysteresis characteristics is effective to a degree in minimizing the error introduced by hysteresis, this material selection technique has introduced a disadvantage in the construction of clamp-on devices. This advantage is that these specifically selected materials generally are of a type having a relatively high magnetic permeability which become saturated at relatively low driving current levels. This characteristic is a disadvantage where substantially large currents are to be measured, such as in the order 500 amperes or more, as it is necessary that the apparatus be operated only in the region where the magnetic core is unsaturated or below the region of the knee of the magnetization curve. Consequently, in order to permit use of the prior art devices utilizing the relatively expensive, high permeability magnetic materials, it becomes necessary to introduce substantial air gaps in the magnetic core circuit to increase the reluctance of the magnetic circuit and enable operation in the linear range of the magnetization curve. Artificial increasing of the magnetic reluctance, however, introduces a further disadvantageous effect in that the magnetic fields have a tendency to then follow air leakage paths. The magnetic field in the gap is thus decreased substantially, resulting in reduced Hall-effect generated output (for the same current input). To overcome the reduced gain of the Hall-effect device and magnetic circuit combination, higher electronic gain is required. Relying on the electronics for increased signal level has the disadvantage of making the total system more susceptible to temperature drift, internally generated noise and radio frequency interference. Also, the current probe useability at low level currents is much reduced, if not eliminated completely. Another undesired result of using large gaps to reduce flux density is a greater susceptibility to external or extraneous magnetic field interference. This interference is manifested as external noise and can be generated by conductors outside the aperture or by ferrous metal near the current probe.